Silver halide emulsions containing antifogging agents



United States Patent 3,244,521 SILVER HALiDE EMULSEQN CONTAINING ANTIFOGGHNG AGENTS Fritz Dersch and Robert J. Clementi, Binghamton, N.Y., assignors to General Aniline & Film Corporation, New York, NFL, a corporation of Deiaware N0 Drawing. Filed Nov. 2, 1961, Ser. No. 149,520 9 Claims. (Cl. 96-66) The present invention relates to photographic materials comprising light-sensitive silver halide emulsions. More particularly, this invention is directed to stabilized silver halide emulsions which have less tendency to fog and which still possess substantially the same light sensitivity.

It is well established that light-sensitive emulsions such as gelatin-silver halide emulsions are subject to fogging. Fogging in general, and chemical fogging in particular, may be defined as a uniform deposit of silver extending over and either partially or wholly obliterating the image and is caused in a number of Ways namely, the nature of the emulsion, its age and the conditions under which it has been stored, the nature of the developer, impurities in the developing solution and the time and temperature of the development. Additionally, chemical fogging is promoted when such active agents as metal are employed in a concentrated solution or are used in the presence of an excess of alkali. Similarly, the oxidation of certain developing agents such as metol and hydroquinone exerts a powerful fogging action.

Thus, it is an obiect of the present invention to provide light-sensitive silver halide emulsions which are stabilized against the formation of fog by the addition thereto of antifogging agents which tend to prevent the formation of chemical fogs and which suffer little or no lowering of light sensitivity thereby.

We have discovered that the above object is readily accomplished by intimately associating with silver halide emulsions pyrazolopyrimidines of the following general formula:

wherein R represents a hydrogen atom, alkyl groups containing from one to eighteen carbon atoms, aromatic groups and halogen and X-represents a hydrogen atom, a carboxyalkyl group or the roup S-A wherein A represents the pyrazolopyrimidine structure shown above.

It has been observed that the antifogging effect is generally due to the presence in the emulsion of the pyrazolopyrimidine structure and not necessarily to the size of configuration of substituents R and X.

Representative substituents wherein R represents an alkyl group include groups such as methyl, ethyl propyl. butyl, hexyl, octyl, decyl, dodecyl, octadecyl and the like. The substituent R can also represent aromatic radicals such as phenyl, naphthyl and anthryl radicals as well as a halogen atom such as chlorine.

The substituent X can represent a hydrogen atom, a halo radical such as a chloro radical, carboxy-allcyl radicals such as carboxymethyl, carboxyethyl, carboxybutyl and carboxyhexyl radicals, and the disulfides corresponding thereto.

The antifogging agents which have been found to exert such beneficial effects to light-sensitive silver halide emulsions for the prevention of chemical fogging can be readily prepared in accordance with known procedures. such as described in J. Am. Chem. Soc, vol. 79, 6407-15 (1957). For example, 4-mercapto-6-methyl pyrazolo (3,4-d) pyrimidine is conveniently prepared by reacting under refiux conditions S-aminoi-cyanopyrazole with acetic anhydride to provide 5-acetylamino-4-cyanopyrazole which is subsequently reacted with potassium hydroxide and hydrogen peroxide to yield 4-hydroxy-6-methyl pyrazolo (3,4-d) pyrimidine. The hydroxide is then converted to the mercapto derivative by reaction with phosphorous pentasulfide.

Beneficial effects in fog reduction are obtained when solutions of the antifoggant are incorporated into the silver halide emulsions as ripening finals or as coating finals. Ripening finals are added during the ripening or the sensitivity increasing stage of the emulsion-making process whereas coating finals are added to the silver halide emulsion just prior to the coating of the emulsion on a suitable support such as, for example, paper, glass or film.

The addition of the antifoggants can be made before, during or after the addition of the soluble silver salt to the soluble halide in the presence of a suitable carrier or colloid such as gelatin, polyvinyl alcohol, solubilized casein, albumin and the like.

If desired, the antifoggants of the present invention can also be employed as components of the developing solution.

The amount of antifogging agent employed as additives to the light-sensitive emulsions varies from about 0.1 to about 50 milligrams of antifoggant per 0.6 mole of silver halide. When the antifogging agents are employed as ripening finals it has been found advantageous to employ them in amounts in the range of from about 0.1 to about 25 milligrams per 0.6 mole of silver halide whereas when the antifogging agents are employed in coating finals it has been found advantageous to employ amounts in the range of from about 0.5 to about 50 milligrams of antifoggant per 0.6 mole of silver halide. Obviously, the development of optimum characteristics within the range set forth above will depend, in part, upon the type of,

emulsion and can be readily determined in each case.

In some cases, it has been found advantageous to apply the antifogging agents of the invention in adjacent layers, for example, in a separate undercoating layer or in the antiabrasion gelatin surface. Alternatively, it has been found advantageous, in some cases, to apply the antifogging agents of the invention in one or all processing baths or in pre and post baths.

It has also been found that the antifogging agents of the invention can be compatibly employed in combination with other known antifoggants and stabilizers; with sulfur-, reduction and 1netal-, and noble metal sensitizers as well as in combination with polyoxyalkylene polyols, their derivatives; polyvinylpyrrolidones and other accelerators.

The following examples will serve to illustrate the practice of the invention:

Example I A silver halide emulsion in gelatin containing 2% silver iodide and 98% silver bromide was prepared in a conventional manner and brought up to its maximum light sensitivity. It was then readied for coating, finals were added such as sensitizing dyes, and hardening agents. A 0.1% solution of 4-mercapto-6-anethylpyrazolo (3,4-d) pyrimidine was prepared and added to the emulsion as an antifoggant and stabilizer. The emulsion samples contained about 0.6 mole of silver halide. The so prepared emulsion samples were coated on a suitable cellulose ester base' and dried. Samples of these film coatings where then G exposed in a Type 113 'sensitomete-r and developed in a developer of the following composition:

The stabilizer used in this example was prepared as follows:

A mixture of 159 grams (1.47 moles) of -amino-4- cyanopyrazole and 500 ml. of acetic anhydride Was refluxed for 11 hours to give a clear, dark solution. Excess acetic anhydride was evaporated on the steam bath under reduced pressure. The viscous residue was triturated with 60 ml. of benzene to give a tan solid. The solid was filtered and recrystallized from water to give 154 grams of 5-acetylamino-4-cyanopyrazole which melts in the range of ZOO-02 C. A mixture of 182 grams (1.21 moles) of S-acetylamino-4-cyanopyrazole, 852 ml. of potassium hydroxide and 1830 ml. of 3% hydrogen peroxide was war-med at 70-75 C. for 30 minutes. The hot solution was acidified with glacial acetic acid and when allowed to stand, a While solid precipitated from the solution. The solid was filtered in hot 10% potassium hydroxide solution and acidified with glacial acetic acid to give 92 grams of 4-hydroxy-6-methylpyrazolo (3,4-d) pyrimidine having a melting point greater than 340 C. A solution was then prepared comprising 7.5 grams (0.05 mole) of 4-hydroxy-6-methyl pyrazolo (3,4-d) pyrimidine and 100 ml. of dry pyridine whereupon there was added in portions grams (0.11 mole) of phosphorous pentasulfide. The mixture became warm and was refluxed for 2.5 hours and poured into 200 ml. of hot water. The hot solution was acidified with glacial acetic acid and on cooling gave an orange-red solid. The solid was filtered and the filtrate evaporated to about two-thirds of its original volume and on cooling gave 3 grams of yellow solid having a melting point greater than 310 C. The solid was dissolved in hot 10% potassium hydroxide solution; treated with Norit A (activated charcoal), filtered and the hot filtrate acidified with glacial acetic acid to give 2 grams of 4-mercapto-6-methyl pyrazolo (3,4-d) pyrimidine.

Example II The procedure used was the same as in Example I except that there was used as the stabilizer 4-carboxymethyl thioether of the 6-methyl-pyrazolo (3,4-d) pyrimidine. The results of the experiment indicated that 80 milligrams of the compound provided the same effectiveness as 8.0 milligrams of the compound of Example I.

The carboxymethyl thioether is prepared by heating equimoleoular portions of the mercapto compound and bromoacetic acid and sufiicient 2 N sodium hydroxide solution to take up the hydrogen bromide split-off during the reaction under reflux for two and one-half hours. After cooling the reaction mixture is acidified and yields the compound.

Example 111 The procedure was the same as Example I except that there was used the disulfide corresponding to 4-mercapto- 6-methyl-pyrazolo (3,4-d) pyrimidine. The results obtained were similar to those obtained in Example I.

The stabilizer was prepared from the mercapto compound by oxidizing it in ethanol solution with hydrogen peroxide.

Example IV Exposed samples of a photographic film were developed .for twelve minutes at 65 F. in a standard metol-hydroquinone developer. Two tests were made, one with the normal developing solution and one with a developer containing 200 m'gs. of 4-mercapto-6-methyl-pyrazolo (3,4-d) pyrimidine to one liter of developer. Sensitometric strips, developed in the normal developer (control) for twelve minutes showed a fog of .30, whereas those strips, which were developed in the developer containing the antifoggant, had a fog of .20.

Various modifications of the invention will occur to persons skilled in the art. Thus, it is evident that in lieu of using the compounds of the examples, any of the compounds mentioned above can be employed with similar results. Therefore, it is not intended that the invention be limited in the patent granted except as necessitated by the appended claims.

What is claimed is:

1. A light-sensitive photographic material comprising a light-sensitive silver halide emulsion, a carrier therefor and an antifogging agent characterized by the general formula:

N K A T/ I II wherein R is the same as the radical represented by R in the foregoing general formula.

2. The material as defined in claim 1 wherein the antifoggant is located in the silver halide emulsion.

3. The material as defined in claim 1 wherein the antifoggant is located in a layer adjacent the silver halide emulsion.

4. A light-sensitive photographic material comprising a light-sensitive silver halide emulsion, a carrier therefor, said emulsion containing, as an antifog-gant, 4-mercapto- 6-methyl-pyrazolo (3,4-d) pyrimidine.

5. A light-sensitive photographic material comprising a light-sensitive silver halide emulsion, a carrier therefor, said emulsion containing, as an antifoggant, 4-mercapto- 6-chloro-pyrazolo (3,4-d) pyrimidine.

6. A light-sensitive photographic material comprising a light-sensitive silver halide emulsion, a carrier therefor, said emulsion containing, as an antifoggant, a compound of the formula:

7. A light-sensitive photographic material comprising a light-sensitive silver halide emulsion, a carrier therefor, said emulsion containing, as an antifoggant, a compound of the formula:

N I IJN Nkl -orr= H;C\N

it it 8. The process of minimizing and preventing fog in light-sensitive silver halide materials comprising a base having a light-sensitive silver halide emulsion thereon, which comprises exposing said emulsion to light and developing the same in the presence of an antifoggant characterized by the general formula:

wherein R is the same as the radical represented by R in the foregoing general formula.

9. A photographic developer containing a developing agent and an antifog-gant characterized by the general formula:

wherein R represents a member selected firom the group consisting of hydrogen chloro, alkyl aromatic radicals and X represents a member selected from the group consisting of hydrogen, carboxyalkyl radicals containing up to six carbon atoms in the alkyl group and a radical having the formula N ll i N wherein R is the same as the radical represented by R in the foregoing general formula.

References Cited by the Examiner UNITED STATES PATENTS 2,935,404 5/1960 Dersch 96109 X 2,952,539 9/1960 Dersch et a1. 96109 FOREIGN PATENTS 701,054 12/ 1953 Great Britain. 716,327 10/1954 Great Britain.

OTHER REFERENCES Robins: Journal American Chemical Society, vol. 78, pages 784-790, 1956.

NORMAN G. TORCHIN, Primary Examiner. LOUISE P. QUAST, Examiner. 

9. A PHOTOGRAPHIC DEVELOPER CONTAINING A DEVELOPING AGENT AND AN ANTIFOGGANT CHARACTERIZED BY THE GENERAL FORMULA: 